Since lithium secondary batteries have been commercialized, the most important object in research and development into batteries is to provide a cathode active material showing excellent electrochemical characteristics including high capacity and long service life. In addition to the above electrochemical characteristics, it is urgently required for a cathode active material to have excellent thermal safety so that a battery system can ensure the safety and reliability even under abnormal conditions such as exposure to heat, combustion or overcharge.
Cathode active materials currently used in lithium secondary batteries include composite metal oxides such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (0<x<1), LiMnO2, etc. Among those, Mn-containing cathode active materials such as LiMn2O4, LiMnO2, etc., have advantages in terms of processes for the preparation thereof and cost needed for the preparation thereof. However, such Mn-containing cathode active materials are disadvantageous in that they show low discharge capacity. On the contrary, although LiCoO2 is a typical cathode active material used in most commercially available batteries by virtue of its excellent conductivity, high voltage and excellent electrode characteristics, it is not cost-efficient. Meanwhile, a Ni-containing cathode active material, LiNiO2, shows the highest discharge capacity among the above-described cathode active materials. However, LiNiO2 are problematic in that it shows rapid degradation in terms of service life and significantly poor high-temperature characteristics compared to other cathode active materials.
The above-described cathode active materials are lithium intercalation compounds whose structural stability and capacity are determined by lithium ion intercalation and deintercalation. As a charge voltage increases, capacity of such a lithium intercalation compound increases, while the compound becomes structurally unstable, resulting in a rapid drop in the thermal safety of an electrode. More particularly, such cathode active materials in a charged state show a rapid drop in bonding force between metal ions and oxygen atoms, when the internal temperature of a battery exceeds the critical temperature due to internal or external factors. Therefore, oxygen is decomposed and liberated from such unstable cathode active materials as shown in the following reaction scheme:Li0.5CoO2→½LiCoO2+⅙Co3O4+⅙O2 
The free oxygen shows high heat-emission property, thereby causing a thermal runaway phenomenon. Further, the free oxygen may cause a highly exothermic reaction with an electrolyte in the battery, resulting in explosion of the battery. Therefore, initiation temperature and heat flow of the reaction, in which oxygen is liberated, should be controlled in order to ensure the battery safety.
In one method suggested for controlling the above-heat flow and initiation temperature, a cathode active material is prepared through a pulverization process and classification process so as to control the surface area of the resultant active material. The average voltage range of an active material having a small particle size is not affected by current density (C rate), because the active material has a large surface area. On the other hand, an active material having a large particle size shows a small surface area, and thus shows an increased surface polarity when it is subjected to high rate charge/discharge, resulting in a drop in average voltage range and capacity.
In order to improve the safety of a cathode active material during charge/discharge cycles, a method for doping a Ni-based or Co-based lithium oxide with a different element was suggested. For example, Japanese Laid-Open Patent No. 12-149945 discloses an active material for improving the quality of LiNiO2, the active material being represented by the formula of LiNixMyCozO2 (wherein M is at least one selected from Mn and Al, and x+y+z=1).
Another method for improving the safety of a cathode active material is based on surface modification of a cathode active material. For example, Japanese Laid-Open Patent No. 9-55210 discloses a cathode active material obtained by coating a lithium nickel-based oxide with an alkoxide of Co, Al or Mn, followed by heat treatment. Additionally, Japanese Laid-Open Patent No. 11-16566 discloses a lithium-based oxide coated with a metal selected from the group consisting of Ti, Sn, Bi, Cu, Si, Ga, W, Zr, B and Mo, or an oxide thereof.
However, the above methods according to the prior art cannot increase the initiation temperature where the surface of a cathode active material reacts with an electrolyte (i.e., the exothermic reaction temperature where the oxygen bonded to the metal in the cathode active material is liberated). Moreover, the above methods cannot decrease the amount (heat flow) of oxygen decomposed by such reactions. Ultimately, cathode active materials according to the prior art cannot improve the safety of a battery.